1. Field of the Invention
This invention relates to a method of producing a thin film multi-layered substrate.
2. Description of the Related Art
Recently, demand for a higher operation speed and a higher integration density of LSIs has been ever-increasing in order to reduce machine cycles in data processing apparatuses such as computers and electronic devices. With such a demand, signal delay of a wiring substrate on which electronic components such as the LSIs are packaged has become a problem for reducing the machine cycles, and demand for a higher operation speed in this packaging system has become stronger.
Therefore, a high density multi-layered substrate using an organic polymer having a small dielectric constant such as a photosensitive polyimide resin for an inter-level insulating layer to improve a signal propagation speed of the wiring substrate has been proposed.
A method of producing a thin film multi-layered substrate according to the prior art is shown in FIGS. 8a to 8c of the accompanying drawings. According to this method, a plating base conductor 6 is first formed on an insulating substrate 5 by sputtering, or the like, and then an electrolytic copper plating wiring 7 is formed by an additive process or a substractive process (see FIG. 8a).
Next, a polyimide precursor 8 which becomes a polyimide resin after heating and curing and has photosensitivity is coated, and via-holes 9 are formed by effecting exposure and development through a photomask having a pattern of the via-holes 9. The polyimide precursor 8 is then heated and cured to provide an inter-level insulating film 2 (see FIG. 8b).
A second layer wiring is obtained by forming once again the copper plating wiring 7 by the processes shown in FIGS. 8a and 8b, and these process steps are thereafter repeated to obtain a thin film multi-layered wiring substrate shown in FIG. 8c.
According to such a method, however, a chemical reaction takes place between the copper plating wire 7 and the polyimide precursor 8 after the wiring 7 is 10 formed, and a reaction product is formed on the copper plating wiring 7. As a result, the resistance value of the via-hole portions 9 becomes extremely high and exerts an adverse influence on signal transmission characteristics.
Japanese Unexamined Patent Publication (Kokai) No. 60-135902 proposes a method which solves the problem described above and comprises the steps of coating a corrosion-resistant conductor film on the copper plating wiring 7, coating, exposing and developing a photosensitive organic polymer material and then forming the via-holes by removing the conductor film. However, this method is not free from the problem that reliability of inter-level connection drops due to side etching that occurs at the time of etching of the conductor film.
Under such circumstances, a polyimide having, for example, a high aspect resolution and low heat expansion coefficient has been proposed in recent years as a photosensitive polyimide effective for forming a high density wiring pattern, and polyimide having a film thickness of about 40 .mu.m (at the time of development) and a resolution capability of a via-hole diameter of 30 .mu.m has been put on the market.
However, this characteristic is for a Cr pattern, and when a Cu conductor is used, a certain specific sensitizer reacts with Cu in addition to the photosensitive polyimide, so that gelation of amic acid itself or a certain reaction occurs and impedes flattening of polyimide, and eventually, development becomes impossible.
To solve such a problem, a method of forming a barrier metal of Cu/Cr or Cu/Ti on an upper layer without using the Cu single substance for patterning may be possible. However, if Cu is exposed on the pattern end surface, the reaction product proceeds to the surface of an object of development during pre-baking, and the same phenomenon occurs in the same way as on the Cu surface. Accordingly, development becomes impossible. Moreover, plating of the barrier metal is extremely difficult by electroplating or electroless plating, electrolytic dissociation of the single substance is difficult, too, and plating is not possible in the case of Cr due to stress and chemical resistance, and in the case of Ti because it is an active metal.
Japanese Unexamined Patent Publication (Kokai) No. 3-5907 discloses chromate plating on the surface of a copper coil pattern in order to protect the copper coil pattern of a thin film magnetic head coil from denaturation and degradation. However, this reference does not definitely describe the chromate treatment, and also does not describe at all the problems occurring due to the reaction between copper and polyimide and a solution for such problems.
To solve the problem of conduction defect resulting from the reaction between copper and the polyimide precursor that occurs when the polyimide pattern is formed on the copper wiring, Japanese Unexamined Patent Publication (Kokai) No. 4-51518 teaches to treat the copper wiring with chromic acid or a chromate, or with a bichromate, before the formation of the polyimide film. However, the reference does not at all teach or suggest to use the bichromate after mixing it with chromic acid or the chromate when such a treatment is carried out.
As described above, exposure of Cu on the pattern end surface becomes an important problem of the photosensitive polyimide process during the formation of a miniature pattern, and metal coating as a barrier involves many difficulties.
Particularly in the case of photosensitive polyimide of a low thermal expansion and thick film type which has a high aspect resolution, selection of the photosensitizers is inevitably limited, In other words, there remains the problem that the additive reacts with Cu of the exposed portion and is likely to induce inferior development.